Research of Zircon Brick for Glass Furnace

Research of Zircon Brick for Glass Furnace

1. Preparation of Zircon Bricks

Table 1 shows the performance of zircon bricks for glass furnace from different brand around the world. From Table 1, it can be observed that there is little difference in the chemical composition of various zircon bricks. Analysis of the mineral phase reveals that the main crystalline phase in all cases is zirconia, and its performance indicators are related to the internal structure of the brick. The performance differences mainly stem from the aggregates used, making the preparation of high-quality aggregates crucial. It is necessary to use well-sintered and structurally dense aggregates. Coarse zircon particles with a particle size greater than 3mm are generally added to improve thermal shock resistance and other high-temperature properties.

1.1 Aggregate Preparation

The crushed raw materials are sintered, crushed, and screened to obtain the ideal aggregate particles. The molding of the raw material is typically achieved through machine pressing, slurry pouring, or isostatic pressing. In this paper, based on the three sintering methods for aggregate preparation, the further study and research on aggregate preparation using the casting method is discussed.

The casting method for molding is based on the principle of decomposition and recombination of zirconia. By using the casting method, the zircon content in the sand can be reduced by 1% to 2%, and there are no specific requirements for the particle size of the sand. However, attention should be paid to the moisture content to prevent furnace spraying during the melting process. No mineralizer is needed for the raw materials, but to reduce flying materials and dust during the melting process, the sand needs to be agglomerated. During agglomeration, organic binders such as AST or phenolic resin should be added as much as possible. The casting temperature is around 1900°C. During casting, sand pits made of zircon sand or sand molds made of zircon sand can be used. It is best to use coarse zircon sand from Australia for making sand molds. After mixing the sand with 4% binder (by weight) and 5% to 7% water, it is manually compacted and molded. After drying and insulation at 110°C for 4 hours, it can be assembled and used after gradual cooling. After pouring into the sand pit, it can be covered with zircon sand and naturally cooled to room temperature, or it can be naturally cooled to room temperature after insulation at 1450°C for 3 to 4 hours. The aggregate prepared by this casting method has a volume density of above 4.3g/cm³, an apparent porosity of less than 1%, and stable high-temperature performance with good erosion resistance.

Table 2 lists the differences in chemical composition of various aggregates. The aggregates prepared by the casting method consist of monoclinic zirconia and glass phase, which are interlocked and form a dense structure without obvious gaps. The aggregates obtained by sintering are mainly composed of zirconia crystalline phase, and the glass phase is mainly present at the bonding interfaces between particles. The aggregates prepared by isostatic pressing and slurry pouring methods are dense as a whole, and after crushing, cleavage is evident. Machine-pressed aggregates, although stable in chemical composition distribution, have a loose structure with particles bonded by point contact, resulting in large gaps at the bonding interfaces.

By using four different methods to prepare aggregates with similar chemical compositions, samples with different densities (including cast samples) are fired at the same temperature. After being soaked in alkali-free glass liquid at 1500°C for 50 hours, the degree of erosion of the samples is shown in Table 3.

From Table 3, it can be seen that the reduction in apparent porosity and low water absorption of the brick body significantly improves its erosion resistance compared to bodies with higher apparent porosity.

1.2 Ingredient Design

Firstly, it is necessary to consider achieving the expected technical indicators for the chemical composition and physical properties of the brick, while also ensuring good usability. In addition to meeting the above technical requirements, ease of production should also be considered. While ensuring the zirconium content, the impact on firing temperature needs to be taken into account. To lower the firing temperature without compromising the technical specifications (practicality and erosion resistance) of the brick, it is advisable to use fewer or no mineralizers that would affect the brick's performance. Compared to alkali and alkali-earth metal oxides, as well as yttrium oxide and manganese oxide, titanium oxide, as a mineralizer, can promote sintering but has a lesser effect on the brick's erosion resistance.

The particle size of zircon is related to its decomposition rate, and the determination of particle size distribution should adhere to the principle of dense packing while having sufficient composition to improve the bonding and sintering of the ingredients. The particle size composition range is as follows: 0-3.5 mm accounts for 2-5%, 3.5-1.4 mm accounts for 43-53%, 1.4-0.1 mm accounts for 15%, and <10 μm accounts for 27-42%.

1.3 Shaping

Comparative studies were conducted on several binders, including pulp waste liquid, water glass, starch, and AST, with sucrose selected as the binder. The prepared mixture is mixed using a roller mill, and mineralizers, water, and organic binders are added simultaneously, requiring thorough and uniform mixing. The mixture is compacted for 24 hours. The shaping is done using a friction brick press, with a loose ratio of 1:1.25, and the samples are dried at 110°C, with moisture controlled below 1% upon entering the kiln. The strength of the dried body is 7-15 MPa.

1.4 Firing

The firing process for zircon brick is similar to the general refractory materials, as shown in Figure 2. The firing temperature ranges from 1500 to 1700°C. Zircon brick starts shrinking at 1300 to 1400°C, with the most intense shrinkage occurring between 1400 and 1500°C, and minimal shrinkage after 1600°C, indicating the end of sintering. Therefore, during firing, it is recommended to slowly heat up from 1300 to 1500°C, and the highest firing temperature can be set at around 1600°C. If impurity content is low or the amount of mineralizer added is small, the firing temperature can be increased to 1650-1700°C.

1.5 Comprehensive Analysis of Sample Development and Determination of Production Process

Aggregate samples produced by machine pressing have a bulk density of 3.58-3.67 g/cm³, an apparent porosity of 20%-23%, and a load softening temperature below 1600°C. Aggregate samples produced by isostatic pressing and slurry casting have a bulk density of 3.77-3.85 g/cm³, an apparent porosity below 18%, and a load softening temperature above 1700°C. Aggregate samples produced by fusion casting have a bulk density of 3.87-4.0 g/cm³, an apparent porosity below 16%, and a load softening temperature between 1570 and 1630°C. However, the fusion casting method has higher costs and greater difficulty in electric melting control. Among the three methods, the creep value of the aggregate samples subjected to a 50-hour hold at 1500°C is 0.2% for both isostatic pressing and slurry casting. Among the four production processes for preparing aggregates, both slurry casting and isostatic pressing can meet the technical requirements of practical applications. Slurry casting has a mature process, skilled operation, and ease of control. On the other hand, isostatic pressing requires a larger initial investment, has higher technical difficulty, and takes a longer time to master the operation. Therefore, using the slurry casting process for producing machine-pressed zircon bricks is more economically reasonable.

2. Development of Products, Comparison of Performance with Similar Domestic and Foreign Products, and Usage Results

After conducting research experiments using an optimized method, the formulation and process route for the experiments were determined. The comprehensive test results of the samples are as follows: Chemical composition: ZrO content 65.28%; bulk density, 3.77 g/cm³; apparent porosity 16.3%; Cold Crushing Strength 109 MPa; Refractoriness Under Load (T0.6), 1700°C; Permanent Linear Change at 1600°C for 4 hours, 0.03%; and Thermal Shock Resistance exceeding 10 cycles. Table 4 compares the performance of SNR products with the performance of several foreign products used in imaging tube glass furnaces.

3. Conclusion

(1) The production of high-quality pressed zircon bricks using the slurry casting method for aggregate preparation is feasible, and using this process route for brick production is reasonable.